Currently, in cooperative relaying, different options are presented for relay selection and transmission of the selected relays to the destination. There are two ways for relay selection. The first one of these is relaying with pre-selected relays and the second is relaying with post-selected relays.
In relaying with pre-selected relays, one or more relays are selected before the source begins transmission. While this selection can be performed only once during the planning stage (fixed relay), it can also be updated at certain intervals according to the mobility in the network and the average channel quality. In practice, these updates are carried out based on the success rate of the communications made previously between the nodes, and they are reported to the required nodes as control information.
In relaying with post-selected relays, all of the nodes receiving signal with adequate quality from the source can be used as relays. In this case, units which will be used as relays are chosen after the source executes transmission. In both relay selection methods, in case of a plurality of relays performing repetition, these relays can transmit to the destination in different channels or in a common channel and time interval.
Various methods are introduced for transmission of the selected relays in a common channel and time interval. The use of distributed space-time coding is proposed to enable the destination to separate the signals sent from the relays in a common channel and time interval. Another method that is proposed is distributed beamforming. In this method, the relays estimate their channels towards the destination and perform transmission in a way that all the relay signals reach the destination with the same phase.
Selecting a fixed relay during the planning stage makes the system dependent on a single relay. It is not possible to reach the diversity gain present in the network using a fixed relay. A certain gain can be achieved by placing the selected relay to geographically advantageous locations. However this generally makes it difficult to reach the selected location. Additionally, placing the relay to a preplanned location is not preferred especially for ad hoc networks and military communication systems.
Methods requiring internodal coordination bring disadvantages in several aspects. The messaging needed for coordination leads to an additional traffic load and additional delay. Furthermore, when the purpose of the coordination is adapting to the channel conditions which changes continuously due to the nature of the wireless communication environment, as in relay selection, from time to time it would not be possible to coordinate rapidly enough to adapt to these conditions, which causes a decrease in the performance.
In pre-selected relaying methods, diversity gain decreases when the source-relay and/or relay-destination channels are weak. Since the possibility of the presence of at least one relay that has strong channels both to the source and to the destination increases in relaying methods with relay post-selection, these methods provide more diversity gain than the relaying methods with relay pre-selection. Another disadvantage of the relaying with pre-selected relays is that it requires coordination between the nodes for relay selection.
Increasing the number of relays that receive sufficient signal from the source and perform repetition increases the diversity gain. Relays performing repetition by transmitting in different channels increases the necessary time or band use for unit information and as a result decreases the end-to-end communication efficiency. In cases where the relays are determined after the source's transmission, since the number of the relays and the channels required for the repetition changes continuously, performing repetition in different channels will further increase the need for coordination.
Efficient operation of the methods which allow transmission of relays in a common channel and time interval, such as distributed space-time coding and distributed beam-forming, can be possible by pre-selected relays. Both of these methods require precise synchronization between the units for performing repetition in the same time interval. Further coordination is needed for fulfilling this requirement. A receiver structure which allows transmission in a common channel and time interval is suggested for use at the destination for relaying with post-selected relays. However operation of this receiver is based on the preconditions that at the destination both precise synchronization and the knowledge of the channel qualities from the relays towards the destination are present. Fulfilling these preconditions also require further coordination.
The United States patent document no. US010260240 discloses a cooperative relaying method and system for asynchronous networks. In the said invention, similar to the invention we are proposing herein, a system for transmitting the source's message to the destination by means of asynchronous relays without needing an additional coordination between the relays is provided. However, for the method suggested in the invention, the block transmission by the relays should be sent with delays of guard time as long as the sum of channel response time and timing error. Furthermore, in the related method, depending on the random selection of the relays, the diversity gain expected to be observed is not the highest gain that can be reached with the system in all cases. In addition to these, the receiver needs to estimate the channel between the relays and itself for the equalization that it will perform in frequency domain. In this document, it is assumed that the channel will not change throughout the data communication and it is asserted that it is sufficient to estimate the channel at the destination with training sequences of length two blocks that are added before the data sequence which are sent by the relays. However the channel may change, depending on the system requirements, such as mobility of the units or frequency hopping, etc. The said invention differs from our present invention disclosed herein in terms of coding, channel estimation and data estimation.
The United States patent document no. US2009316763 discloses diversity and capacity gain by randomized codes in distributed cooperative communication systems. In the said document, each relay divides the data it receives into parallel streams, encodes with multiple-input multiple-output (MIMO) coding methods and forms the transmission signal by computing the random linear combinations of the generated new streams. Similar to the present invention disclosed herein, the relays transmit to the destination unit in a common time interval and channel. However, use of multiple receiver antennas is required in the destination unit for operation of the invention disclosed in the said document. As it is also stated in the document, the number of receiver antenna should be at least as high as the number of the relays. First of all, use of multiple antennas increases the unit cost and may cause low performance in units having small sizes. The invention disclosed in the document requires the number of data streams generated in each relay to be less than the number of relays. Satisfying the conditions for the numbers of antennas, relays, and streams, requires coordination among the relay and the destination units, and thus brings an additional communication burden, in the system. The present invention disclosed herein may work with units with multiple antennas but does not require more than one antenna in any unit. Furthermore, the receiver unit does not have to know the number of relays.